The invention relates to a method for manufacturing a semiconductor device, and more particularly to a method for fabricating a metal oxide semiconductor (MOS) transistor.
Increasing degrees of integration of semiconductor devices have resulted in a dramatic reduction in the effective channel length of MOS transistors in such semiconductor devices, causing many problems due to the short-channel effects. A number of techniques, for example, techniques associated with the formation of recessed channels, have been proposed to increase the effective channel length of the devices without an increase in the degree of integration of the devices. However, there is a limitation in increasing the degree of integration of devices while ensuring the effective channel length of the devices.
Due to increased concentration of a dopant in a source region and a drain region of MOS transistors of a semiconductor memory device, particularly dynamic random access memory (DRAM) devices, the intensity of an electric field applied to the source and drain regions is gradually increasing. As a result, the level of a voltage, at which a punchthrough phenomenon is caused wherein a depletion region of the source region is connected to that of the drain region, is lowered to cause malfunctioning of the DRAM device. The increased electric field enables the occurrence of hot carriers, leading to a degradation in the electrical properties of the device. Further, the increased electric field causes a leakage current, leading to a deterioration in the refresh characteristics of the device.
Halo ion implantation has been employed to avoid these undesirable phenomena. According to halo ion implantation, a source region and a drain region doped with a high-concentration dopant of a first conductivity type, e.g., a p+ dopant, are counter-doped with a dopant of a second conductivity type, i.e. an n-type dopant, to decrease the concentration of the dopant in the source and drain regions, thus achieving a reduction in the intensity of an electric field applied to the source and drain regions.
However, the halo ion implantation using counter doping may cause a decrease in the concentration of the dopant in the source region and the drain region, resulting in a degradation in the driving current characteristics of devices. It is also known that known ion implantation techniques have a difficulty in reducing an electric field at a particular depth through local concentration control. Thus, halo ion implantation may result in a reduction in the total concentration of a dopant in junctions of a source region and a drain region, thus achieving a reduction in electric field but causing unwanted degradation of device characteristics.